For the life of me, I can't fathom why Jerry....

Divorce wasn't so easy then, especially due to social pressures. And Jerry would almost certainly have lost custody of Tina to his wife, which was the last thing he wanted. Even more importantly, Jerry had a code of honor, an ideal of how a man in his position should act, and he just wasn't going to toss that away lightly. To some it might seem unrealistic by present standards, but that was his basic character, and it made him all the more appealing to Charlotte.